The present invention relates to high performance data transceivers and, more particularly, to a predriver for driving the input of a high speed output driver.
Data transceivers are used in a variety of computer systems and communication networks for transmitting data between various components of the system. As data throughput requirements continue to increase, data transmission frequencies continue to increase. For example, output drivers having a low output voltage swing, such as Gunning Transceiver Logic (GTL/GTL+) drivers, have been developed for driving high performance processor system buses and computer system backplanes. A basic GTL+ output driver includes a low impedance open drain N-channel metal oxide semiconductor (NMOS) transistor, which drives the receivers attached to the system bus through an incident wave switching interface. The NMOS driver transistor drives the incident wave with sufficient amplitude to switch the logic states of the receivers. External pull-up resistors are used to terminate the system bus transmission lines and to provide a pull-up current path when the NMOS driver transistor is off.
A predriver circuit is typically used to drive the input to the NMOS driver transistor at a high frequency. A traditional GTL+ predriver circuit is configured as a logic NOR gate, with additional circuitry to speed the rise and fall times of the predriver output. At high frequencies, it is difficult for the predriver output to maintain an full "rail-to-rail" voltage swing. In particular, it is difficult for the predriver to drive its output completely to zero volts before the output is charged back up again with the next data transition. At higher frequencies, the low voltage output of the predriver can rise close to the threshold voltage of NMOS driver transistor. The NMOS driver transistor may not turn off completely, which can result in undesirable leakage current through the transistor or unintended switching at the receivers. If the predriver output discharges more quickly, thus reaching a lower voltage, the faster discharge can cause greater overshoots and ringbacks at high frequencies.
Improved predrivers which are capable of driving an output voltage rail-to-rail within the desired frequency while controlling overshoots and ringback are desired.